stabilizing slopes: sri lankatcdc2.undp.org/gssdacademy/sie/docs/vol12/13sri lanka.pdf · ods for...

Stabilizing Slopes: Sri Lanka

GENERAL INFORMATION

� Implementing Institution: Department of Geology,University of Peradeniya

� Head: Mr. L.R.K. Perera

� Details of Institution: Address: Department of Geology, University of Peradeniya, Peradeniya 20400, Sri Lanka

Tel.: (+94) 81 238 9156, 239 2011

Fax: (+94) 81 238 8018

E-mail: [email protected]

Website: www.pdn.ac.lk/sci/geology, www.soiltechltd.com

� Implementation Period: Soil nailing was introducedinto Sri Lanka in the late 1990s. The current project, startingin 2004, will take two years to complete

� Costs: The project, granted through the Sri Lankan RoadDevelopment Authority (RDA), supported by the AsianDevelopment Bank, and linked with the private companySoil Tech Ltd., was allocated US$287,500 for the constructionof soil-nailed walls covering approximately 3,300 squaremetres of cut slopes along the Gampola-Nawalapitiya andGampola-Nuwara Eliya roads.

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178 VOLUME 12: EXAMPLES OF NATURAL DISASTER MITIGATION IN SMALL ISLAND DEVELOPING STATES

SUMMARY

Slope failures causing landslides are com-mon in many parts of Sri Lanka, particu-larly along the major roads. Typically,conventional roller-compacted concreteor rubble-masonry structures have beenused to mitigate such events but thesemethods have drawbacks, including thelength of time taken to construct suchstructures. Therefore, alternative meth-ods for stabilizing hazardous slopes needto be investigated. Among the alternativeapproaches are soil nailing, whichinvolves the insertion of steel rods into anunstable slope to pin it in position, andsoil bioengineering, which involvesspraying a mixture of grass-seeds, otherseeds and fertilizer onto a suitable matrixso that when the seeds germinate, theroots hold the underlying soil in place.Both methods can help to reduce con-struction times so that work can be com-pleted before the onset of the annualrainy season. Soil-nailing and soil-bio-engineering methods were introducedinto Sri Lanka by Soil Tech Ltd., a privatecompany.

Initially, in the late 1990s,Government officials provided Soil TechLtd. with a length of road on which todemonstrate the methods. During thisperiod, the technical team developed thetechnology by introducing innovativeitems using locally available or importedmaterials. Local vendors have now beeneducated about the specifications of thematerials required.

Following the success of this demon-stration project, the Road DevelopmentAuthority (RDA) agreed to the currenttwo-year project in which 3,300 squaremetres of cut roadside slopes would be soil nailed along the Gampola-Nawalapitiya and Gampola-NuwaraEliya roads.

For the new project, a technical teamwas formed that comprised representa-tives from Soil Tech Ltd., the Universityof Peradeniya Department of Geologyand the project director from RDA. Theteam was responsible for educating thevendors and supervising activities.

BACKGROUND

AND JU S T I F I C AT I ON

Small- to large-scale slope failures causedby gravity acting upon a destabilizedlayer of loose rock that rests on thebedrock occur frequently in Sri Lanka(fig. 1). In many instances, such land-slides have been induced by heavy rain-fall. However, man-made activities havealso caused slope destabilization in manyparts of the country. For example, in thecentral highlands, roads often experiencesuch failures and urban construction inthe region may also damage slopes.Typically, once destabilized, remedialmeasures include scraping away and dis-posing of any loose material without anysignificant engineering measure beingtaken to prevent further failures. In addi-tion, in the recent past, large-scale tec-tonic plate movements, micro- to macro-seismic activity, certain land-use activities

Stabilizing Slopes: Sri Lanka 179

and infrastructure developments havecaused the destabilization of slopes inmany parts of Sri Lanka.

Particularly in the case of infrastruc-ture development, there is a need forproper cost-effective and user-friendlymeasures to curb slope destabilization.Proposed highways in the central high-lands and southern regions of the countrywill require additional attention sincethere are plans to operate them as tollroads. Many small hydrological projectsinitiated in the central highlands, includ-ing the construction of reservoirs, mayalso require containment of the slopesand embankments to cope with thepotential destabilization of the sites.

Since the slopes in hilly areas aremoderate to high, with angles rangingfrom 10 to 60 degrees, small- to large-scale slope destabilization is commonaround many cities in the central high-lands. In such areas, there is a high risk oflocal slope failure, especially duringheavy rain. These failures may eventuallypropagate downhill, endangering proper-ty and installations along the wholelength of the slope. Once a surface failuredevelops or a major slope failure is initiat-

ed, remedial measures are very expensive.It is therefore of great importance to pro-vide efficient and effective measures toidentify and to support compromisedslopes before any adverse slope failureoccurs.

Conventionally, roller-compactedconcrete or rubble-masonry structureshave been used to stabilize slopes. Thesemethods require a large area at the baseof the slope and a long construction time,both of which create inconveniences.They are also time-consuming to con-struct and expensive as they require alarge amount of building materials.Depending on the slope and the pressureof the water in the soil pores, these struc-tures last from 20 to 30 years, after whichtime, cracks start to appear. At this vul-nerable stage, owners are unable tochoose inexpensive replacement or reme-dial actions. The invention of soil-nailingand soil-bioengineering methods, whichare slowly being accepted by the publicin Sri Lanka as viable alternatives to con-ventional slope-stabilization methods,now allows more effective and less costlyremedial measures to be installed oncompromised slopes.

Figure 1 One of themany landslides that haveaffected private propertyand public installations inSri Lanka.


DES C R I P T I ON

Soil nailing has been used for the past 30years in a wide range of civil engineeringprojects and has been found to be veryeffective in stabilizing slopes that havebeen damaged by certain land-use activi-ties and infrastructure developments aswell as by large-scale tectonic platemovements and micro- to macro-seismicactivity. The technique originated as anextension of rock bolting and the “newAustrian tunnelling method”, developedin the early 1960s, which combines bolt-ing rocks together and using shotcrete, atype of concrete that has a smaller maxi-mum aggregate size (usually 10 millime-tres) and that is sprayed onto rock sur-faces at high velocity.

When carrying out engineering con-struction in soils with low load-bearingcapacities or in destabilized soils subjectto lateral movements and land subsi-dence, Sri Lankan engineers and geolo-gists thought that a new methodologyshould be introduced to cope with con-struction difficulties. In addition, the con-struction of basements in areas close tocoastal towns and the stabilizing ofslopes associated with buildings and minihydroelectric power stations in the cen-tral highlands require an effective, low-cost method based on lightweight struc-tures to stabilize the slopes.

To this end, soil nailing was intro-duced into Sri Lanka in the late 1990sand subsequently adapted to the localconditions and to make use of locallyavailable materials. Both simple soil nailing

and soil nailing coupled with an afford-able earth-retaining structure and eithervegetation or a shotcrete facing over theunstable slope were proposed in order toarrest potential landslips. The centralhighlands and marshy areas close to thecoast of Sri Lanka also need special atten-tion since many new buildings, roads andharbours are being constructed.

TH E SO I L -N A I L I N G CO N C E P T

The basic concept of soil nailing is toreinforce and strengthen the existingground by inserting relatively closelyspaced inclusions, usually steel bars,which are also referred to as “nails”, into aslope or excavation as it proceeds fromthe top downwards. This process createsa section that is internally stable and ableto retain the groundmass behind it (fig.2). The nails are passive in nature anddevelop their reinforcing action throughinteractions between the nail and theground into which they have been insert-ed as the ground deforms during andimmediately following construction.

The nails, which typically are 16 to25 millimetres in diameter, are installedinto drill holes with diameters rangingfrom 50 to 100 millimetres and spacedbetween 1.5 and 2.5 metres apart. Thelength of the nails depends on the geom-etry of the wall and the spacing betweenthe nails as well as soil, groundwater andload conditions. Usually, the upper nailsare longer than the lower nails, consistentwith the stress distribution with depthand the distance to the critical slip surfaces. Nails are inserted at an angle of


15 degrees below the horizontal to facil-itate grouting (fig. 2). A recessed grid orfull cover facing typically made of shot-crete reinforced by steel bars or weldedwire mesh is also usually added to furtherstabilize the slope (fig. 3).

Soil nails typically consist of steelreinforcement inclusions and may be cat-egorized on the basis of their method ofinstallation and degree of corrosion pro-tection. Corrosion protection is usuallyprovided by galvanizing or by epoxycoating in addition to the protection pro-

vided by the covering of grout or filling.For conventional drill and grout nailinstallations, the nail grout consists of aneat cement with a water-to-cement ratioof about 0.4:0.5. For economic reasons, acement grout containing sand is also usedto fill larger nail holes.

TH E BI O E N G I N E E R I N G

CO N C E P T

Bioengineering uses the binding capacityof the roots of such plants as grasses and

Figure 2 The concept ofsoil nailing. Note that theload varies along the lengthof the soil nails and that theactive zone at the top of thenailed slope reaches into thesubstrate approximately 0.3to 0.35 times the height ofthe slopeHeight

Resistant zone

Distribution oftension along nail

FacingActivezone

0.3 - 0.35x height

Figure 3 A slope that has been treated by soilnailing and then coveredwith a shotcrete facing.(This was the first soil-nailed

wall in Sri Lanka and has been

standing now for more than

11 years.)


trees to stabilize slopes and hillsides.Different methods utilize plants in combi-nation with constructions of wood, stoneor wire and include planted pole walls, liveslope grids, live wooded crib walls, vege-tated stone walls and vegetated gabions(wire mesh cages filled with rocks).

The selection of the most appropriatebioengineering method depends on suchfactors as the position of the slope, thestate of the ground and the availablematerials. For centuries, wood and livingplants were the only materials availablefor hill and slope stabilization work.Recently, however, traditional techniqueshave been modified and applied onceagain to curb soil erosion along slopes. Inthese cases, living material such as stumpsprouts, branches and deciduous trees are used.

Bioengineering can be a relativelyquick procedure, depending on themethod selected. However, it should benoted that cuttings and rooted plantsshould be used only during the dormantseason. During the growing season, it isbetter to use sods of grass and otherspecies that will quickly develop theirroot systems.

Two methods being tested byUniversity of Peradeniya and Soil TechLtd. staff are dry seeding and hydroseed-ing, which involves spraying a mixture ofseeds and fertilizer onto exposed areas ofsoil. Seeds are then protected with coirmats and a mixture of straw and compost.After reseeding, the slope can be furtherstabilized by transplanting stump sproutsof deciduous trees.

Figure 4 shows the final outcome of abioengineered slope immediately afterthe completion of the soil-nailing struc-ture. The upper part of the slope hasbeen treated solely by bioengineering,whereas the steeper, lower part of theslope has been reinforced with soil nailsand a concrete mesh with grass growinginside it. Within this whole framework,man-made structures cover 10 per cent ofthe ground surface while the remainder iscovered with bioengineered natural soils.Recent trials involving only the use ofgrass and stump sprouts of Gliricidia, a treeof the Fabaceae family, show promise fortotal bioengineering control of slopes.

Bioengineering can also be relativelyless expensive than soil nailing and othertechniques. In Sri Lanka, costs are estimat-ed to be around US$70 per square metre.

Figure 4 A soil-bioengineered slope treatedwith coir matting, seeded with grass and withyoung trees planted for long-term stability.


Through improvements in design and theproper management of technical crews, itmay be possible to reduce costs further.

DE V E L O P I N G T H E ME T H O D

Initially, the scientific team from theUniversity of Peradeniya and Soil TechLtd. aimed to introduce the soil-nailingmethod as an innovative approach forhighly urbanized areas in the capital,Colombo, and Kandy in the central highlands. The plan was then to expandthe technique to other areas of Sri Lanka.However, in the late 1990s, when soilnailing was being introduced into SriLanka, local people were not convincedby the method in its original form. Thescientific team, therefore, developed amore user-friendly low-cost method. Theteam members were then awarded a smallproject that entailed stabilizing a slopewhere no other slope stabilizationmethod was practical. In 2004, Soil TechLtd. received a larger construction assignment that involved stabilizing cutslopes along roads in the central high-lands. The Road Development Authority(RDA) needed to stabilize the slopes

within a prescribed time frame and thesoil-nailing and soil-bioengineeringmethods were considered the onlyoption for certain locations.

Based on these and other experi-ences, the scientific team has now devel-oped state-of-the-art technology byintroducing several innovative pieces ofequipment using locally available materi-als or imported materials that are readilyavailable on the local market. For exam-ple, the iron nails are now coated or gal-vanized before use and plastic or ironspacers are attached along the length ofthe rod. Drilling equipment has also beenmodified so that it can be used at differ-ent angles and different heights, both ofwhich improve the efficiency of thedrilling process. The main advantages ofthese modifications are that they can beapplied rapidly and they use material thatis available locally. The team is nowequipped with competent experts,trained staff and specially designedequipment that enable it to undertakesoil-nailing and soil-bioengineering pro-jects in Sri Lanka and elsewhere (fig. 5).

Figure 5 A vertical slope cutthrough a basement that is undergoing soil-nailing treatment.


The soil-nailing method involves:

1. Preparation of the slope;

2. Drilling at prescribed intervals;

3. Insertion of nails with end capsand small devices to keep the nailcentralized in the drilled holes;

4. Insertion of cement grout to bondwith the soil;

5. Construction of a continuous,welded, wire-reinforced mesh overthe slope;

6. Fitting of the nails with steelplates and studs;

7. Covering of the slope with plasticmesh to curb local erosion; and

8. Covering of the welded wire meshwith shotcrete and the rest of theslope with either shotcrete or abioengineered green facing.

Recently, instead of plastic mesh, acoir-based net has been used in tandemwith the bioengineering method (fig. 4).These textiles, which are available locallyand made from a by-product of thecoconut industry, are more environmen-tally friendly and have proved to beeffective in making aesthetically beautifulslopes with a covering of grass.

Soil nailing can also be used as a temporary measure to protect wallsbeyond basements being built intosteeply sloping land or during the con-struction of high-rise buildings close tovertical slopes. In both situations, verticalfaces are often prepared although theyare vulnerable to collapse during con-struction. Such collapses have been known

to bury both equipment and labourers.Temporary soil nailing using short ironrods can be used in such situations.

Landslide hazard maps prepared bythe National Building ResearchOrganization can also be consulted toidentify areas at risk. In any case, adetailed geological examination and asubsequent geotechnical engineeringappraisal are needed before designing the exact soil-nailing method to be usedin a particular case.

ED U C AT I N G LO C A L VE N D O R S

Soil Tech Ltd. personnel were welltrained in the drilling operations andinstructed local vendors in the field inhow to cut the slope and carry out step-wise drilling. The horizontal drilling wascompleted with a newly designed drillingmachine and the mechanics attached toSoil Tech Ltd. were responsible for train-ing people in the mechanical activities.During pilot work conducted in the highlands, the technical team from theUniversity of Peradeniya trained the vendors in the geological and soil-engi-neering aspects of the soil-nailing processand designs. Training was also providedfor those who were in the field for a shortperiod. The technical team’s long-termgoals include a fully fledged programmethat covers both classroom (specificallydesktop training) and field training forvendors. This has not been accomplishedyet, but plans are being made to put thisprogramme into action.


PAT EN T I NG AND

COMMERC I A L I Z AT I ON

Soil Tech Ltd. has applied for local patent-ing rights to use soil-nailing and bioengi-neering techniques in Sri Lanka. The com-pany has also allocated funds to supportmarketing initiatives aimed at making theproducts commercially viable and avail-able to the public. This has been achievedmainly through exhibitions and a public-awareness campaign run in collaborationwith the University of Peradeniya.

PARTN E R SH I P S

Owing to the technical nature of themethods being applied, this was a uniqueexperience that required a partnershipbetween the private company, Soil TechLtd., and researchers at the University ofPeradeniya. University staff supportedthe innovative ideas and carried outresearch, for example, on the stressesexperienced by soil nails of differentlengths in different situations and thebest plant species to use in different bioengineering circumstances. The management component of Soil TechLtd., on the other hand, dealt with theimplementation of the techniques inpractical situations.

The RDA and Pacific Consultantswere responsible for ensuring adherenceto strict quality-control and quality-assur-ance practices.

REP L I C A B I L I T Y

Both soil-nailing and soil-bioengineeringmethods are applicable in many situa-tions and should be considered alongwith other mitigation methods for pre-venting landslides. However, in order forthe techniques to be applied correctly,the appropriate technical expertise mustbe available to make an accurate assess-ment of the slopes in order to select themost effective treatments.

POL I C Y IMP L I C AT I ON S

In Sri Lanka, at the national, regional andlocal levels, the policy of putting projectsout to tender and accepting the lowestbid, owing to financial constraints on thepart of the public body, can create prob-lems. Government practice in allocatingsuch projects to the lowest bidder shouldtake into account not only financial constraints but also the ability of thecompany to deliver the desired outputs.

On a more general economic-devel-opment level, there is very little supportfor small and medium-sized enterprises inSri Lanka. Loans to innovative businesses,such as Soil Tech Ltd., are often providedat very high and unpredictable rates ofinterest, often up to 22 per cent, so thatentrepreneurs have difficulty paying themonthly instalments. Without supportfrom the central bank and theGovernment, small and medium-sizedenterprises, which can be a driving forcefor national development, will be unableto develop to their full potential.


I M PAC T

The project was successful on economic,social and environmental grounds.Conventional rubble-masonry structuresare meant only for short embankments asthe taller the structure is, the larger thearea that needs to be cleared at its basefor the support structure. Even gabioncages, 1.5 metre cubes of wire mesh filledwith rocks and that can be stacked likebricks, need extensive preparatory meas-ures. In contrast, it has been demonstrat-ed that soil nailing can be carried outwithout building such remedial structuresand without making extensive excava-tions at the site, thus reducing the cost ofthe procedure.

Under this initiative, soil-nailingprojects were also completed within ashort period and at significantly lessexpense compared to conventional meth-ods of stabilizing slopes. In Europe andthe United States, cost savings of 25 to30 per cent have been recorded while inSri Lanka cost savings were between 25and 40 per cent. Typically, the percent-age saved increases as the height of theslope increases.

Although the method was most likelyto be used to repair damaged slopes afterconsiderable damage had already beencaused to a property, the soil-nailingmethod is effective where the long-termmaintenance of a compromised slope isconcerned. For instance, compared toother methods, the longevity of slopesstabilized by soil nailing is increased. Asthe nails begin to deteriorate, for

example, both they and the facing caneasily be replaced, which is not possiblefor other slope-stabilizing methods.

To date in Sri Lanka, soil nailing hasbeen used for stabilizing a significantnumber of slopes without disturbing theoriginal land and Soil Tech Ltd. is currently the leading agency in the country for carrying out such work.

L E S SON S L EA RN ED

Lessons learned during the imple-mentation of the soil-nailing processinclude:

• In order for the soil-nailing tech-niques to be applied correctly, theright technical expertise must beavailable to make an accurateassessment of the slopes in orderto select the most effective treat-ments. For example, a geologistand/or an engineer must be pres-ent at the site to decide how toovercome the unnecessary collapseof the slope in the event of heavyrainfall, surcharge on the slope,groundwater flow, condition of thesoil and other environmental fac-tors. When it is necessary to buildvertical walls at a site, it is particu-larly important for construction tobe closely supervised.

• As mentioned earlier, the currentpolicy of putting projects out totender and accepting the lowestbid owing to financial constraintson the part of the public bodyshould be changed as a result of


the problems that often follow.When bidding for projects, forexample, many local companiesunderestimate the costs that willbe incurred in their eagerness toobtain the project, which results in the company facing a financialcrisis part-way through the imple-mentation of the project. As aresult, the project is often leftincomplete. In the case of slopestabilization, this can do moreharm than good. Governmentpractice in allocating such projectsshould take into account not onlyfinancial constraints but also theability of the bidding company to deliver the desired outputs.

• Timely intervention is necessarywhen using soil nailing to preventfurther deterioration of a damagedslope. During the current project,the team often found it difficult toorganize a labour force to com-plete the assignment. In addition,there were many occasions whenthe contractor faced difficultiescaused by cutting the entire slopeduring the preparation procedurerather than making the step-wisecut advised by geologists andengineers. Safety was also compro-mised after heavy rainfall at sever-al sites and, although contractorswere supposed to complete thework before rainfall, the topmostmaterial sometimes collapsed. Inresponse, the team, headed by anengineer and a geologist, immedi-ately summoned the labour force,supervisors, drillers, safety special-ists and other relevant authorities

to explain to them the outcome ifsteep-cut slopes were left open tothe elements during the rainy sea-son. Rain can also slow down theconstruction phase since it is diffi-cult to erect the scaffolding struc-tures needed as a platform for thedrilling and shotcrete processes.

• To counter people’s scepticismabout the durability and longevityof the soil-nailing method despiteits availability in Sri Lanka sincethe 1990s, the team discoveredthat allowing people to examine acompleted soil-nailing construc-tion helped them to accept newmethods. The conservative natureof society in Sri Lanka means thatnew approaches must be intro-duced carefully if they are to bewidely accepted by the community.

FUTUR E P L AN S

Current research is focused on theidentification of the most suitable plantsfor bioengineering. Rather than “planti-ng” steel rods, as in soil nailing, bioengi-neering offers the opportunity to plantgrasses, rooted plants, stump sproutplants and trees, the roots of which helpto stabilize the slope. The longevity ofbioengineered slopes may also beincreased compared to that of soil-nailedslopes as the plants can be replaced moreeasily over time. So far, Gliricidia trees,various invasive plants and severalclimbers and creepers have been selectedfor further trials. It is estimated that, byincorporating bioengineering techniques,


the cost of stabilizing a slope can bereduced by some 20 per cent.

In addition, at present, if a slopeaffecting a property is in need of remedi-ation work and timely action is necessaryto prevent further damage and additionalcosts, then the owner of the property,whether private or public, must cover thecost of the work. The scientific team istherefore holding discussions with localbanks in an effort to develop a suitablemechanism for supporting the communi-ty in such times of need.

P U B L I C AT I O N S

Fernando, G.S.K., Jayasena, H.A.H. and Perera, A.G. (2001). Use of soil anchors in stabilizing a steep slope: Acase study. International Conference onInnovative Techniques in Earth Retainingand Ground Improvement Systems.Institute of Engineers Sri Lanka Centre,Central and Northwestern Provinces,Kandy, Sri Lanka.

Jayasena, H.A.H. (1994). Reconnaissancesurvey of Ratnapura-Godigamuwa landslide. Report presented to the United Nations Educational, Scientificand Cultural Organization Man and theBiosphere Project. Natural ResourcesEnergy and Science Authority of Sri Lanka. 12 pages.

________. (2001). Lightweight geomaterial for construction activities in Sri Lanka: The pros and cons.Proceedings of the Expanded Polystyrene System 2001, Salt Lake City, Utah, United States.

Case Study Prepared by:

H.A.H. JayasenaAddresses: Department of Geology,University of Peradeniya, Peradeniya 20400, Sri LankaTel: (+94) 81 238 9156, 239 2011Fax: (+94) 81 238 8018 E-mail: [email protected], Soil Tech Ltd., # 4/22, Pitakanda Road, Anniewatta, Kandy, Sri LankaTel: (+94) 81 222 4174, 11 286 1818, 11 286 1271Fax: (+94) 81 222 4174 E-mail: [email protected]

Project Participants:

C.J. Gamage, managing director, Soil Tech Ltd.: Introduced innovativesoil-nailing techniques applicable to Sri Lankan conditions and helped toredesign the equipment using locallyavailable material.

A. Gopalasingham, geotechnical engineer, Soil Tech Ltd.: Design andimplementation of soil-nailing activitiesin Sri Lanka.

Densil Aponso, project director, RoadDevelopment Authority: Provided criticalinputs for the improvement and effectiveimplementation of the project.

J.R.B. Cusack, engineer, PacificConsultants: Ensured the adherence to thorough quality-control and quality-assurance practices during the field application of soil-nailing and bioengineering processes.

stabilizing slopes: sri lankatcdc2.undp.org/gssdacademy/sie/docs/vol12/13sri lanka.pdf · ods for...

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